Molded metal parts, and vapor phase sintering process, molds and compositions for preparing same



June 4, 1968 R. c. REED 3,386,121 MOLDED METAL PARTS, AND VAPOR PHASEslNTlXING PROCESS, MOLDS AND COMPOSITIONS FOR PREPARING SAME Filed June20, 1966 4b 3o 2q 27 INVENTOR.

24a www L25 L26 y my United States Patent() MOLDED METAL PARTS, ANDVAPOR PHASE SIN- TERING PROCESS, MOLDS AND COMPOSITIONS FUR PREPARINGSAME Robert C. Reed, Poway, Calif. (S90 Armada Terrace, San Diego,Caiif. 92106) Filed June 20, 1966, Ser. No. 558,875 13 Claims. (Cl.75-224) ABSTRACT F THE DISCLOSURE The disclosed process uses metal vaporof a selected group of metals to simultaneously sinter and alloy a massof loose metal powder shaped and heated in a poro-us, nonmetallic,non-oxidizing mold, thereby to form in one step a molded and alloyedarticle having the desired final shape. No vapor-forming metal is incontact with said loose metal powder, so that only the metal vapor actson said powder. The vapor-forming metal is selected from the groupconsisting of magnesium, zinc, cadmium, arsenic, rubidium, cesiu-m,potassium, sodium, and mixtures thereof. The loose metal powder isselected from the group consisting of iron, cobalt, nickel, manganese,titanium, zirconium, columbium, vanadium, copper, silver, gold, alloysin which one of the foregoing elements constitutes the base, compositepowders, and mixtures of the foregoing.

This invention relates to (l) molded metal products, parts yor piecesprepared from metal powder(s) with the aid of metal vapor such as zincor magnesium vapor, (2) the process for preparing said products, (3) themolds used in said process as `well as the process for preparing saidmolds, (4) and certain metal compositions used in and/ or resulting fromsaid molding process.

In brief description, the molded products and parts of the invention areprepared by 'lling a mold cavity with metal powder(s) without applyingmore than slight pressure, placing the so-filled mold in a retortcontaining a measured amount of vaporizable metal (eg. zinc, magnesium,etc), substantially closing the retort (by preference a small ventopening is provided), heating the substantially closed retort above thevaporizati-on temperature of the vaporizable metal, thereby to permeatethe atmosphere of the retort with said metal vapor, cooling the retortand contents sufficiently to prevent oxidation of the metal contained inthe retort, and finally recovering the resulting molded metal part(s)from the retort. For this purpose, the molds used in providing anddefining the mold cavities for the sought parts are prepared from carbonand are purposely made in such a way as to be quite porous, whereby themetal vapor gains easy access to the metal powder(s) contained in themold cavities. Also the molds are bafiied with vapor-impermeable baffleswhich serve to properly guide and distribute the metal vapor evenly inreference to the mold cavities. The molded parts prepared in accordancewith the invention can lbe prepared by starting with any elementalmetal(s) and/or alloy(s) (and with or without composite powders) meltingabove the temperature at which the metal vapor is generated andsustained. However, I prefer the metals grouped in the Periodic Tableand known as the heavy ductile metals, and which commonly include, iron,cobalt, nickel, copper, silver and gold. I also prefer for some purposesmixtures of copper or iron with nickel. I especially prefer to startwith nickel powder where the resulting molded parts are intended forjewelry usage because of the excellent color, polish, and corrosion andtarnish resistance exhibited by such molded parts when the finishedalloy is binary and contains 32-35% zinc, balance nickel, or when thefinished alloy is ternary and contains about 11.2- 11.5% zinc, balanceequal weights of nickel and iron.

3,386,121 Patented June 4, 1968 lCe Accordingly, the objects of thisinvention are:

(l) To provide novel molded metal articles by starting with -selectedmetal and/or composite powders;

(2) To provide a novel process for preparing said molded metal articles;

(3) To provide novel porous molds adapted for use in the vapor-sinteringprocess of the invention; and

(4) To provide certain novel metal compositions which are particularlyadapted to the purposes of the invention.

The `foregoing and other related object-s of the invention will beunderstood more fully from the following description of the invention,taken in conjunction with the attached drawings in which FIG. I is avertical sectional view of a typical assembly used in forming a mold ofthe kind employed in practicing the invention;

FIG. II is an exploded vertical sectional view of a typical assembly ofmolds and mold parts in a retort of a type adapted for the 'commercialpractice of the invention, certain vertical dimensions being exaggeratedfor clarity;

FIG. III is a vertical sectional view of a typical heating furnace andretort assembly suitable for the practice of the sintering process ofthe invention;

FIG. IV is an isometric view of a typical jewelry item molded inaccordance with the invention, the view specifically illustrating anornamental plate carrying the initial T; and

FIG. V is a vertical sectional View of a typical pack assembly referredto in Example 9, the vertical dimensions of the drawing beingexaggerated for clarity.

The invention will now be explained in connection with FIGS. I, II andIII of the drawings, it being understood that such explanationillustrates only one particular and preferred embodiment of theprinciples of the invention, with nickel being used to represent anyappropriate starting metal powder and zinc being used to represent anyappropriate vaporizable metal. Referring now to FIG. I, whichillustrates a preferred manner of preparing a mold, a flat base plate 1of porous carbon (prepared in the general manner about to be describedin connection with the mold itself) is placed on any suitable flatsupporting surface and one or more patterns 2, 2 are rested on its uppersurface in any desired arrangement and disposition. A metal rim mem-ber3, typically of iron and of a size, conformation and thickness adequateto define a cavity within which the desired mold can be formed is placedon said base plate 1 so as to surround the patterns 2, 2 and so as toleave room for forming a mold surrounding each and all of said patterns.The cavity so dened by said base plate 1, said rim member 3 and saidpatterns 2, 2 is then filled with a plastic mixture 4 composed ofpowdered carbon (preferably powdered graphite) and hardena-ble butinitially normally-liquid organic resin binder. The physical andchemical attributes of said binder will be described in more detailhereinafter, but for the present purposes the binder can be identified(solely for purposes of illustration) as an acid-activated furfurylalcohol resin. Said mixture of powdered carbon and activated liquidbinder (illustratively composed of about 3 parts by weight of powderedcarbon and l part of binder) is rammed into the cavity in any suitablemanner (i.e., by tamping it in by hand or by ramming it into place witha mechanical hammer or conventional ramming press). After the cavity hasbeen filled with said mixture, the top surface is struck off with ablade so as to be smooth and at the same time level with the top surfaceof rim member 3. A flat porous carbon plate 5 having lateral dimensionssmaller than those of rim member 3 is placed on said smooth uppersurface of the rammed mixture in such position as to be within Ibut outof contact with rim member 3. Another porous carbon plate 6,substantially a duplicate of plate 1, is then placed on top of plate 5with its edge in vertical alignment with the edge of plate 1. Ifadditional molds are to be made, then plate 6 can be used as the baseplate for the next assembly, and what has been described above can berepeated until any desired number of molds has been prepared, thusyielding a columnar stack of unit assemblies having a predeterminedperipheral conformation. If only one mold is to be prepared, then plate6 simply serves as a top or cover plate for the unit.

Plates and 6 are used to maintain the mixture 4 (and the resultant moldwhich it forms) in a flat condition during the next step of firing themold unit(s) to carbonize the binder. It will be understood that wheretwo or more mold units have been stacked in the manner described, thenplates 5 and 6 function in like manner in all units.

The next step in preparing the -molds .is to heat them to an elevatedtemperature at which all of the resinous binder is carbonized. It willbe understood that some time after the powdered carbon/ binder mixturehas been rammed to form a mold, the normally-liquid binder, due to thepresence of suitable activator (hardener, catalyst, converter) becomesconverted to a solid condition. Hence the rammed mixture becomes hardand stiff, and it can then be separated from the patterns, if such isdesired. However, for commercial practice of the invention, I prefer notto so separate the molds from the patterns and instead prefer to usepatterns which have been made from a material which can be dissipatedfrom the mold units during the carbonizing firing. Thus patterns made ofparaffin, waxes, synthetic resins or other thermally-decomposable andevanescent material can be so used. Considering now that patterns 2, 2are made of such a material, the single mold unit of FIG. 1, or a stackof mold units corresponding thereto, is next disposed on the cover of amoderately tight-fitting can. The can itself, having a small ventopening in its bottom, is slipped down over the mold unit or stack ofunits and joined with its cover, thereby enclosing the mold unit orunits. As will be understood from the subsequent description of FIG.III, the can and cover used for this purpose can be one of the retortsused in the practice of that part of the invention. The filled can isnext placed in any suitable heating furnace wherein it is heated tocarbonizing temperature (eg. about 1200 F. where the binder is saidactivated furfuryl alcohol resin). Dense black smoke exits from the ventopening in the can during the carbonization period and heating should becontinued until the issuance of said smoke ceases. The can, stillclosed, can then be removed from the furnace and allowed to cool to roomor other convenient handling temperature, at which time the can isseparated from its cover and lifted away to expose the fired mold unitor units. Each unit is then carefully disassembled, primarily for thepurpose of removing the rim `member 3 but the disassembly also providesan opportunity for inspection of the carbonized mold and in thecommercial practice of my molding process it is conveniently combinedwith (a) the step of filling the mold cavities with metal powder and (b)the step of reassembling the filled molds for subsequent treat-ment withmetal vapor (i.e., retorting or vapor sintering).

Referring now to FIG. 'II, which illustrates a preferred way ofpreparing an assembly for retorting, there is shown a thin-walled metalretort 7 and tight-fitting cover 8. The assembly of molds and baffleswhich is to be enclosed within the retort is built up by starting withthe inverted cover 8. A porous carbon plate 6 is first laid on thecover. Said plate can be one of those removed during the disassembly ofcarbonized mold units described above in connection with FIG. I. Thus asthe carbonized mold units are being disassembled, one of the plates 6, 6can be used to start the assembly which is to be built up for retorting.The plate 5 of the carbonized -mold assembly is not used and is laidaside. Then the carbonized mold itself is removed and is ready forfilling with metal powder. For this purpose it is simply rested on asupporting surface and metal powder (eg. fine nickel powder, minus 325mesh) is poured into each cavity and levelled by drawing a straight edgeacross the surface of the mold. If this operation fails to remove allexcess metal powder, the slight amount which remains can be ignored. Thefilled porous mold is then ready for insertion into the assembly ofparts for the retort. However, before it is added to said assembly, athin, fiat, ferrous metal bafiie plate 9 is laid on plate 6 and itssurface is covered with a thin layer of graphite powder 10 (e.g. minus35 mesh). Then a layer 11 of zinc powder (eg. 20 mesh powder)representing a proper weight of zinc in proportion to the weight ofnickel powder in the mold cavities (a minimum ratio of 1:20 should beobserved and preferably the ratio is between 1:20 and 1:1) is sprinkledon the layer 10` of graphite powder. The zinc layer 11 is in turncovered with a thin layer 10a of graphite powder. These layers (9, 10,10a and 11) form a porous, slightly compressible bed adapted (a) tosupport the mold which is next to be added to the assembly, (b) todistribute the supply of zinc fairly uniformly across the bottom face ofthe mold, and (c) to provide a carbon/zinc blend that will avertoxidation of the zinc and reduce any zinc oxide present while t-hefinished retort assembly 1s being heated up to the vaporizingtemperature of the zinc.

The filled mold 12, described above, is now placed on top of -graphitelayer 10a. Then a thin layer 13 of powdered graphite (eg. minus 35 mesh)is sprinkled or sifted over the entire upper surface of said filledmold. Next, another thin flat ferrous metal balile plate 14 is laid onsaid layer 13. This completes one unit as built up for retorting. If aplurality of filled molds is to be retorted together then said unit canbe repeated the desired number of times thereby to build up a stack ofthe desired size. The retort 7 is then placed over the mold unit orunits and joined at its lower edge with cover 8. It will be understoodthat the retort 7 and cover 8 are dimensioned so as to be snug-fittingand substantially completely filled by the unit or units being enclosed.That is, the aim here is to have very little air remaining within theretort, most of it being displaced by the said unit or units. Thus theheight of the retort will vary with the number of mold units built upover cover 8 unless it is preferred to use a retort of uniform size andto substitute a column of graphite plates to fill that part of theretort not occupied by molds. Also it will be appreciated that by makingthe molds initially with dimensions yielding a snu-g fit in retort 7, nodifficulty arises in this connection when the mold units are being builtup for retorting. Another reason for having a snug fit between theretort and all parts of the mold unit(s) is that such a fit promotesuniform and rapid heating of the unit(s) when the retort is fired byexternal heating.

Referring now to FIG. III, a filled retort prepared as last describedabove is represented by reference character 15 and is shown in firingposition on the hearth 16 of a furnace chamber defined by walls 17, 17and top 18. While optional, I prefer also to place a roof 21 over top 18but spaced therefrom by spacers 22, 22, thereby to control the furnacedraft. Heat can be generated in any desired way around and about retort15, but I prefer to use fuel gas/ air burners 19, 19 pointed so as toapply their flames tangentially to the inner walls of the furnace and soheat the retort 15 by radiation. This method prevents direct impingementof the flame on the retort and prolongs the life of the latter. Suchburners not only give rapid heating but can also be adjusted so as togive ame which is nouoxidizing or reducing in character, as desired. Byso adjusting the burners, the useful life of the retorts can beprolonged. If oxidizing fiame, or electric heat (eg. resistance orinduction) generated in the usual oxidizing at-4 mosphere is employed,the thin low cost expendable type metal retorts can easily be burnedthrough before the metal vapor therewithin has had full opportunity tocombine with the metal powder in the mold cavities. By main tainingreducing reducing conditions around the retort,

such burning through is eliminated and the thin-walled retorts can infact be used a number of times before being discarded. Those skilled inthe art will recognize that a protectively-reducing atmosphere aroundthe retort can be established and maintained in various ways other thanby incomplete burning of a combustible fuel/ air mixture. However, Iprefer the latter because of its rapid heating rate and ease ofmanipulation and adjustment. Nevertheless, my objectives can be achievedequally well with other kinds or types of furnaces, as will be obviousto those skilled in the art.

During the retorting, the retort and its contents are brought up to thevaporization temperature of the vaporizable metal being used, e.g. zinc,and preferably are brought to a slightly higher temperature (l550-l750F. for zinc as the vaporizable metal used to vapor sinter nickel, iron,copper or equal mixtures of nickel with copper or iron). Duringretorting, some small amount of metal vapor exits through the vent holeand produces a readily identifiable flame. I purposely make the venthole quite small (eg. .060 in diameter) so as to minimize said loss ofvapor, but the hole is not eliminated because the flame of vapor servesthe very useful purpose of indicating that satisfactory conditions thenprevail within the retort. Also the ebbing of the flame can be used toindicate that the pre-weighed Zinc supply is exhausted and that thenecessary amount of vapor has been supplied to the metal powder in themolds, thereby assuring uniformly reproducible results from oneretorting to the next. Thermocouple 23 is positioned in the llame.

As will be understood from the description of the way in which the unitsof each mold assembly are prepared prior to retorting, the supply ofvaporizable metal needed to react with each cavity-full of metal powderis located out of physical contact with the loose metal .powder but onlya short distance away from the latter. Hence little time is needed afterthe vaporizable metal has reached its vaporization temperature and afterthe selected peak furnace temperature has been reached. In other words,no soaking time is needed after the vapor iame begins to diminishproviding the retort is then at peak operating temperature. If thetemperature of the retort is below peak when vaporization of measuredzinc is complete, heating is continued and the temperature raisedcontinuously until the desired operating maximum is attained. Cooling ofthe retort is begun immediately when both vaporization of the zinc iscomplete and the molds are at the desired operating temperature.

Since the mold assemblies (units) consist almost entirely of carbonand/or graphite, they have excellent thermal conductivity. My experiencehas established that the assemblies heat quite uniformly throughout.Suliice it to say that I have yet to observe any evidence ofunderheating at the centers of the molds due to thermal gradients, oncethe retort has been heated sufiiciently to produce the tell-tale vaporflame at the vent hole. As for time-attemperature needed to effectpenetration of metal vapor to the center of the masses of metal powderin the mold cavities themselves, I have found that the vapor willpermeate each cavity-full completely in thicknesses up to 1A by the timevaporization of the set amount of zinc is complete. This7 of course,occurs automatically when the temperature of the molds reaches 1663 F.,the boiling point of zinc at 760 mm. Hg pressure. The action ofsintering in the powder mass as the result of the alloying of the twophases (powder and vapor) is intensified when the interactiontherebetween produces constituents which are liquid at or below theboiling point of zinc. For instance, in the case of copper powdersintered with zinc vapor, solid copper will dissolve more than zinc attemperatures well below that necessary to boil zinc at atmosphericpressure. Therefore, temperatures leading up to 1663 F. will causelimited liquefaction of each copper particle, as it transforms to brass,and consolidation is thereby elfected more thoroughly.

I have observed that an improper balance of zinc vapor and maximumoperating temperature will result in complete melting of the resultingbrass and a consequent loss of shape and dimension of the part occurs.'When proper amounts of vapor are supplied by the simple expedients ofof exhausting a specific amount of zinc at, or in the time taken toreach, a specilic peak temperature, sintered brass parts of uniformquality are produced. Depth of penetration 0f the zinc vapor is easilymeasured in the cross section of a molded part made from copper powdersince the color of brass is easily distinguishable from that of copper.With other starting metal powders, color may be no criterion, andpeneration can best be determined by chemical analysis of samples takenat various depths within the finished part. For the reasons given, Iprefer to practice the invention by bringing the retort to aternperature of 1675 F. in no less than three hours, using a furnacedimensioned as shown in FIG. III and starting with the furnace chamberat room temperature. This period of heating allows uniform and thoroughsoaking of the molds and metal powders, and eliminates the need for anyholding period at l675 F. Longer heating times can, of course, be usedfor special purposes or for thick masses of metal powder. However, fornickel base jewelry items, where the molded articles seldom havethicknesses over about 1A, a three hour heating cycle at l750 F. hasbeen found to be adequate for a retort containing 15 pounds of nickelpowder and l5 pounds of mold weight when starting with a cold furnacechamber. This gives finished articles that are still porous yet have:been thoroughly integrated by the metal vapor into structures which arestrong, malleable and sonorous. More description of the finishedarticles of the invention is given hereinafter.

After the desired heating cycle and temperature have been attained, theretort is allowed to cool and measures should be taken during suchcooling to prevent oxidation of and early failure of the retort with aconsequent oxidation of the metal articles within the retort. To thisend the burners 19, 19 can be turned to their lowest levels givingstable flames which are smoky. Thus little heat is generated and areducing atmosphere is provided. Further to this end the opening in roof1S of the furnace can be reduced in size (as by inserting a plug) so asto be just large enough to vent the products of combustion of theburners. Another way to effect protective cooling is to plug saidfurnace opening so as to leave a pin hole opening, and turn orf the airsupply t0 the burners. The burners can then be adjusted to maintain aflame outside the furnace just above said pin hole opening in the plug.This ills the furnace with combustible fuel and prevents influx of air.

After the retort has cooled below about 800 F. further protectionagainst oxidation is no longer needed. Hence the retort can be removedfrom the furnace and allowed to cool in the ambient atmosphere untilcold enough to handle. It can then be opened and its contents removed.All the porous carbon plates can be saved for reuse. The moldsthemselves, however, are expendable since in most instances where thefinished articles are of intricate design and articles are held firmlywithin the mold cavities and the molds must be broken to release them.Some attachment of the articles to the cavities is strictly a mechanicalone and is due to the initial porosity of the molds. Little pin pointsof metal penetrate the pores and serve to anchor the articles to thecarbon. The carbon represented by the broken molds can be saved andeventually reground for further use. The attachment of the articles tothe carbon occurs mainly when the graphite powder used in preparing themolds is 35 mesh. Molds made of screened graphite powder passing a meshscreen do not adhere to fiat surfaces of molded parts but do adherewhere in contact with line details in design. This part of the mold canbe easily removed from the metal article by wire brushing. Sometimes themolded articles have thin webs of metal attached to them (resulting fromincomplete removal of excess metal powder from the mold surfaces afterthe cavities were filled in the lirst instance). Such metal ashing canbe broken off easily. The molded articles so recovered in a clean butessentially as-molded condition exhibit a microroughness resulting fromthe initial porosity of the molds. This microroughness is easily removedby sanding, filing, grinding, etc., after which the articles can besmoothed and polished. FIG. IV illustrates the finished articles removedfrom the molds of FIGS. I and 1l; as far as dimensions are concerned thefinished as-molded articles are true replicas of the patterns exceptthat they exhibit a slight shrinkage in all dimensions. This is due tothe shrinkage of the molds themselves during the carbonization firing.

The process for making my molded articles having now been described inconnection with a particular embodiment, the following generalizationsconcerning the process 4can be made:

(l) Raoults law applying to dilute solutions states that the vaporpressure of a solution is lower than that of the pure solvent by anamount that is proportional to the concentration of the solute. The termsolvent generally refers to a liquid but more broadly it can mean thecomponent of a gaseous, liquid or solid mixture that is present inexcess of all other components in the system. As an example, the vaporpressure of an ordinary solution, in which water is the solvent, islowered 170 mm. Hg for each gram of solute in 1000 g. of water and up totwo or three times this amount for non-ordinary solutions. For solventsother than water a characteristic value for each is observed. Thisdecrease in vapor pressure can be expressed another way by saying thatthere is an increase in the temperature at which the vapor press-ure ofthe solution represents that of the pure solvent. Corresponding to thiselevation in temperature, there is an increase in the temperature atwhich the vapor pressure of the solution will exceed an externalpressure of one atmosphere or 760 mm. Hg. This constitutes a rise in thenormal boiling point of the solution.

When two or more miscible metals form an alloy as a solid or moltensolution, Raoults law may be applied and the partial pressures FA and pBof the metals and in the binary system are approximately equal to:

PAZPANA and PB=P BN B where:

PAzvapor pressure of pure metal A PB=vapor pressure of pure metal BNA=mol fraction of metal A in the alloy NB=mol fraction of metal B inthe alloy When metals show a tendency toward immiscibility, the partialpressures show a positive deviation from Raoults law by increasing,Hargreaves (The Vapor Pressure of Zinc in Brasses, Iourna Institute ofMetals, 64, 115, 1939) has shown that additions of aluminum effectivelyincrease the vapor pressure of a brass of a given zinc content 4by amarked amount. Zinc and copper when combined in a simple binary alloyundergo a mutual decrease in activity and their vapor pressures show avery negative deviation from the law as they become much less as abrass. The extent of the deviation increases as the molal ratio N A/NBbetween the metals increases and indeed it has been reported byHargreaves that the vapor pressure of solid brass is much lower than thevapor pressure of pure zinc, the amount being related to the coppercontent. These fundamental findings `to be described again in thefollowing passages were seen to be directly applicable to themetallurgical conditions prevailing in the course of the process of thepresent invention.

The low boiling point of such an abundant and useful -mctal as zinc hasprompted the establishment of its vapor pressure over a wide temperaturerange. With a vapor pressure varying from one mm. Hg at 910 F. to 760mm. Hg at 1663 F., zinc has very high vaporization rates and this uniqueproperty has led to the development of a number of processes utilizingthe feature.

The vaporization of zinc has long been of interest, particularly in themetallurgy of brass ever since Roman brass was made by heating copperwith charcoal and a zinc carbonate ore to red heat in a closed crucible.Metallic zinc reduced from the ore by the charcoal vaporized anddiffused int-o the solid copper. When the melting point of the resultantbrass was depressed sufficiently, the alloy liquilied and flowed intothe Crucible bottom. Recently Hargreaves reported in his study of thechanges occurring at the surface of solid brass during annealing thatthe vapor pressure of zinc in 60-40 `brass (60% Cu) was one mm. I-lg at1085" F. By comparing this value with one given previously for pure zincit can be seen that the 60-40 alloy can be heated 175 F. higher thanpure zinc before vaporization from the brass reaches the order shown byzinc alone. The vapor pressure of an `alloy of Cu and 30% Zn was foundby Hargreaves to be one mm. Hg at 1150" F., serving to furtherillustrate the effect of copper, which is practically non-volatilecompared with zinc, in depressing the vapor pressure of zinc in thealloy.

From these data it can be reasoned that if zinc is heated in closeproximity to copper powder and the formation of oxides of the metalsav-oided, at approximately 900 F. and above it is to be expected thatalloying will take place between the vapors of zinc and the particles ofcopper. Then, since the vapor pressure of zinc in the alloy initiated bythis reaction is depressed by the presence of so large an amount ofcopper in relation to the zinc vapors formed, a reversible vaporizationof zinc is prevented and the alloying action continues unabated as thetemperature of the process is increased steadily to the boiling point ofthe Zinc where the last of the liquid phase evaporates and the supply isexhausted. At this point, if the supply of zinc was pr-operly gaged toresult in the formation of a brass analysis that will not be subject togross melting at the boiling point of zinc and slightly above (l675 F.),a reproducible sintered brass article of uniform shape is produced.

To further illustrate the fact that little zinc loss from the brass ispossible by a reversible reaction, we can compare the vapor pressure of760 mm. Hg of pure zinc at its boiling point with data compiled byHargreaves showing the actual vapor pressure of zinc in a `brasscontaining copper to be 12 mm. Hg and in an 84% copper brass to be 25mm. Hg at the same boiling point temperature of l663 F.

The data of Hargreaves have been presented to show that certain basicassumptions can be made concerning the process of sintering copperpowder by the use of vaporized zinc. It can be further reasoned that asalloying continues during the range of heating needed to liquefy zinc at787 F. and to boil it at 1663 F., each copper particle becomes a brassparticle of increasing size, and by diffusion, of increasing uniformity.As size increases, the contact area between particles enlarges adding toand strengthening the consolidating effect. With proper control of zincand by limiting the temperature of the process to that needed tocomplete vaporization of the zinc, the formation of a liquid phaseconsistent with copper-zinc equilibrium conditions can be utilized togive a limited amount of liquid phase sintering to further densifysintered copper articles without the loss of shape or dimensions.

Adding to the evidence already furnished to support the theory that zincutilization is reasonably efficient, I have the results of a zincvapor-sintering test made on a single 1A thick mold carrying 51/2 oz. ofcopper powder in the form of fifty 1A par-ts. Two oz. of zinc werecharged with the mold and a peal; temperature of l55G F. was applied.

The initial weight of copper powder, transformed to yellow brassarticles by the sintering, gained 1.75 oz. This corresponds to a 24%zinc addition to the alloy. A peak temperature of 1550" F. has been usedfor Virtually all zinc vapor sintering of a copper powder. Vaporizationof zinc is complete at this temperature and monitoring of the cycle isdone exactly as it is when higher temperatures of 1675 F. and above areused to produce redder brasses by adding less zinc to the initialcharge. A very important consideration, as will be seen, is that ofallowing enough time for vaporization to proceed to completionregardless of the temperature used.

The use of zinc vapor to sinter nickel requires even less attention todetails than necessary to vapor-sinter copper powder. Temperatur-es usedare higher than for brass because 1900" F. represents the solidus for a50% zinc-50% nickel alloy, and if substantial liquid phase sintering isdesired, this temperature needs to be reached unless an alloycomparatively high in zinc is desired. For the latter, an excessiveamount of zinc can be charged and the resulting alloy produced at lowertemperatures compatible with the Ni/ Zn equilibrium diagram which showsthe solidus continuously depressed by zinc addition. In my work withnickel I have found a balance of zinc and temperature that producesarticles of plea-sing color showing neither the pinkish cast of nearlypure nickel nor the dull gray of high zinc alloys. The temperatureadvised for nickel is 1750 F.

In the case of nickel, I find it easy to accept the theory of vaporphase sintering that I set forth above for the alloying andconsolidation of copper. It is conceivable that any nickel particle iscapable of alloying with enough zinc furnished in the vapor state toproduce a resultant binary alloy of a moderately low temperature soliduswhich would begin to liquefy first on its periphery and later if at allat its center as diffusion progresses. Certainly, at some stage of theheating, the continued rise in temperature and increase in zinc contentof the alloy will produce some subtle and invisible melting that is notidentifiable in the final structure. The extent of bonding andconsolidation of the nickel particles seems to warrant this sort ofconclusion.

Hargreaves, in other work, measured the vapor pressure of an alloy ofnickel, copper and zinc and reported a sub- `stantially lower vaporpressure than in the simple binary brass alloys. He states: one nickelatom has the same effect on the depression of the vapor pressure of zincas have approximately 1.3 atoms of copper. Thus the loss of zinc from analloy of nickel and zinc containing a lhigh ratio of nickel would benegligible even at temperatures in excess of 1900 F., assuming it wouldbe necessary to go higher.

(2) As noted, any metal and/or composite powders melting above thetemperature used to vaporize the vaporizable metal and capable ofalloying with it can be used as starting powders. The antecedent processused to prepare said powders is not critical. The starting powders canbe of any desired particle size and particle size distribution,including formulations which have been graded carefully to give optimumpacking. For jewelry, bearings, filters, etc., powder of mesh and finerwould normally be used. For specialty items, however, coarser powderscould be appropriate. For jewelry in particular, I prefer to use finepowders, e.g. minus 200 mesh and preferably 325 mesh and finer powders.Such fine powders produce finished articles having microporosity whichis not visible to the unaided eye. As noted previously, mixtures ofdesired metal powders can be used. For example, while iron powdersproduce molded parts which are serviceable, the parts still oxidize inmoist atmospheres. By mixing iron and nickel powders together, e.g.equal amounts by weight, such rusting even in marine atmospheres can beavoided. I have tested jewelry items, made by vapor-sintering nickelpowder with zinc vapor, by exposing same to marine atmospheres for longperiods of time and have found no evidence of corrosion or tarnish ofthe silver-like finish except on mirror-polished surfaces, which showslight dulling. A finely ground or brush finish shows no change inluster or color. Pre-alloyed powders can be used in practicing theinvention. However, I have found that in some instances they reactsomewhat less favorably with the integrating metal vapor than docorresponding mixed elemental powders.

(3) While I prefer to use zinc as the vaporizable metal when practicingthe process described above, magnesium vapor and mixtures of magnesiumand zinc vapors can be used succesfully in retorts which are atatmospheric pressure, as can other metals such as cadmium, mercury,arsenic, rubidium, cesium, potasium and/ or sodium. While I presentlyprefer atmospheric retorting because of its relative simplicity, itshould be understood that the invention can be practiced by using closedretorts which (a) have been evacuated to any desired sub-atmosphericpressure, or (b) have been pressurized with inert gas such as nitrogen,argon, helium, etc., to any desired super-atmospheric pressure. Thus,operating temperatures and pressures can be correlated to the conditionsneeded to operate with the selected vaporizable metal(s) and/or startingpowders.

(4) The porous molds, plates, etc., described above can be made of anydesired form of carbon, e.g. coke, charcoal, graphite, coal, etc.However, I presently prefer graphite because it is commerciallyavailable in grades having known purity, and because it does notdeteriorate in my process and hence can be reused with a minimum of riskand uncertainty as to its operating behavior.

(5) The normally-liquid organic resinous binder(s) used in preparing myporous molds and mold parts can be of widely divergent chemical nature,as can be the agents which convert the normally-liquid materials to asolid state. The term organic as applied to these components of myprocess should be understood, of course, to exclude the organometalliccompositions, since such materials leave metal-containing residues whencarbonized. Thus for illustration only, I mention the following types orclasses of organic resins: phenol/aldehyde resins, a1- kylatedphenol/aldehyde resins, amine/aldehyde resins, alkylated amine/aldehyderesins, alkyd resins, oleoresinous materials, maleic-type unsaturatedpolyester resins, unsaturaed-alcohol-type unsaturated polyester resins,epoxy and other oxirane oxygen types of resins, terpene resins, vinylresins, polyurethane resins, styrenated alkyds, low-to-moderatemolecular weight polyolens, etc., including mixtures of the foregoingwith each other and/ or with natural resins, gums and waxes. Theconverting agents, are, of course, nonuniversal and function selectivelywith particular resins only. They include such materials as oxygen ofthe air, mineral acids, organic acids, alcohols, aldehydes, ketones,peroxides, etc. It will be understood, `of course, that the resinousbinder(s) which I employ constitute a minor and insignificant feature ofthe invention since they are merely used to carry carbon in a form whichcan be liberated in-situ during carbonization, thereby to generatecarbon as the real and ultimate binder for my starting graphite powders.Thus the aim is simply to so manipulate the starting powder that afinished mold can be secured therefrom which is both porous and composedessentially of nothing but carbon. Any organic, carbonizable binderwhich serves `these ends is therefore appropriate, and is hereincontemplated.

(6) The molds and their contained metal powders, enclosed as describedwithin an impermeable retort containing a supply of vaporizable metal,can be heated by any suitable external heating means, e.g. electric,mixtures of oxidizable fuels plus oxidizer(s), solar heat, etc. Asnoted, however, I prefer fuel gas mixtures since such heating means canprovide rapid heating, constitute a source of heat not affected by metalvapor, and can furnish a controllable atmosphere which at choice can benon-oxidizing or reducing.

(7) Cooling of the hot retort and its contents after generationtherewithin of the permeating metal vapor can be effected in anysuitable manner, eg, by slow, controlled rate of cooling, quenching inappropriate 1iquid(s), cooling by jets of gas or liquid, and/'orcombinations thereof. It will be understood that my carbon molds andmold parts exhibit good thermal shock resistance and hence can be cooledrapidly without destruction. However, where quench cooling is employedthe rcstors should of course be constructed of metal or alloy adapted towithstand a number of rapid cooling cycles without deterioration, unlessit is desired to use retorts which are expendable and are discardedafter each cycle of use, eg. very thin (0.010 thick) low carbon steelcans, clay, or like retorts. Retorts made of essentially nonperviousgraphite can be used to contine the metal vapor, with or without anouter protective container or coating designed to keep the graphite fromburning away too rapidly.

(S) What has been said last above applies equally well to the cooling ofthe cans containing the molds and mold parts which need to be subjectedto the carbonization firing.

(9) While one manner of filling mold cavities with metal powder has beendescribed above, other ways of iiliing can of course be practiced. Itshould be understood that according7 to my principles the metal powderin the mold cavities is not in a compacted condition (i.e. is in nosense analogous to a briquette) and instead is in a loose,non-integrated form. Neverthless, it should be understood thatvariations of the principles are possible and sometimes appropriate. Forexample, one can partially ll a mold cavity with loose powder, theninsert a briquetted core member of desired shape and finally ll allremaining space of the cavity with loose powder. Alternatively, one canpartially till a cavity with one kind of powder and then till theremaining space of the cavity with a dilierent kind of powder, therebyto secure a laminated structure. More than two different kinds of powdercan of course be used similarly, e.g. to secure three or morelaminations composed of elemental powders, or to secure a plurality oflayers some of which are composed of elemental powders and others ofwhich, if desired, can be composed of mixed, pre-alloyed and/orcomposite powderts). Again, solid metal inserts (which are not melted inthe process) can be combined with loose powder for producing reinforcedand/or partially fabricated units.

THE MOLDED METAL PARTS As noted above, my molded parts, althoughsomewhat porous, are firmly integrated metallic structures. A moldedstrip, for example l x 8H x 1AG, is flexible, resilient and sonorouswhen struck rather than giving the dull nonsonorous thud characteristicof usual sintered metal parts. My parts can be coined, rolled, forgedcold to some extent, and otherwise shaped and densified by mechanicalworking. lvletallographically they exhibit the characteristics of truealloys, most easily identifiable when the starting metal powders areliner than about 325 mesh. With coarse powders, the conversion of thestarting particles to alloy depends on the amount of time permitted fordiffusion of the vaporized metal into said particles as well as on theamount of vaporized metal made available to the particles. Thus withcoarse starting powders or a small charge of eg. zinc, cores ofunmodified starting metal can be found by examination of a polishedcross section of the vapor-sintered part while the outer portions showthe expected alloy structure of equi-axed grains which seemingly are oihomogeneous composition. When starting with minus 325 mesh copperpowder, a treatment with c g. zinc vapor at l550 F. converts all thestarting particles to alloy. A polished cross section of the sinteredpiece exhibits the appearance and color which is characteristic ofbrass. By starting with line nickel powder, a treatment with zinc vaporat 1750 F. converts the particles to alloy. However, in this instancethe colors of nickel and zinc are so similar that unmodified startingparticles of nickel are ditticuit to identify by color alone. When apolished section is observed in gross its bright metallic luster is veryclosely similar t0 that of silver and hence is noticeably dilirent fromthat of nickel or zinc. The alloy also exhibits good resistance tocorrosion and tarnish, as already pointed out above. The color remainsvery attractive (a) after long exposure in contact with the skin, and(b) after long exposure in contact with most foods, beverages, commonmedicinal ointments, skin creams, lotions, etc., and hence the alloy iswell adapted for use in jewelry items.

It should be understood that my molded parts need not be polished to beattractive in appearance, particularly for jewelry and like usage, sincevery attractive pieces can be prepared by simply sanding certain areas,thereby achieving contrasting appearances. For example, a name plate oran initialled belt buckle can be molded in the manner described aboveand linished by sanding only the top surface of each letter. Theso-sanded surfaces exhib-it a bright metallic appearance while theremainder of the item provides a dark, lusterless background. lt will beunderstood that said dark background appearance results from themicroroughness of the as-molded surfaces, and as much or as little ofthis as is desired can be retained for its decorative appeal. ln otherwords, for some purposes my as-molder pieces can be used as such, or canbe given only little tinishing Thus finishing operations can be held toa minimum, if desired.

For non-jewelry usage my molded parts can be given accurate as-moldeddimensions and hence can be made for a variety of end usos, eg. machineparts, decorative hardware items, parts for electrical equipment,objects of art, etc.

PARTICULAR ALLOY S AND METAL CGMPOSITtGNS As pointed out hereinabove,the starting weight ratio of: volatile metal(s) to starting powdershould be at least 1:20. At the other cxtreme, ratios of 100 to l andhigher are possible, but of course these high ratios give finishedarticles which are composed mostly of the vaporized metal(s). I preferto operate with ratios between 1z20 and 1:1 and as noted especiallyprefer the even narrower ratios between 1:4 and 1:2, inclusive.

For jewelry and allied items made from nickel and zinc, I particularlyfavor the production of (a) finished parts composed essentially of thebinary nickel/Zinc alloys containing about 20-35% zinc, balanceessentially all nickels, or (b) finished parts composed essentially ofthe ternary nickel/iron/zinc alloys containing about 5- 15% zinc,balance essentially all iron and nickel in about equal parts of each byweight. I especially prefer such ternary alloys containing aboutlll-11.5% zinc. {ou/ever, another ternary alloy which I presently likefor some jewelry items is that composed of copper/nickel/ zinc.

COMPOSITE POWDERS A development arising out of the wet precipitationmethods for producing metal powders is the manufacturing of so-calledcomposite powders. These powders consist of a core material coated withe.g. nickel. It is possible to envelope the core material completelywith nickel, provided a coating at least 1-2 microns thick is deposited.Composite powders presently are predominantly finer than 325 mesh. Somecore materials which have been coated with e.g. nickel include iron,copper, chromium graphite, tungsten, tungsten carbide, phosphorous andaluminum. The first obvious advantage of such powders is that materialswhich are readily oxidized can now be made a part of my sinteredarticles, eg. materials such as aluminum, phosphorous, tungsten, andchromium. AS exemplilied hereinafter, the carbidcs such as tungstencarbide can also be included as starting metal powders in the practiceof the present invention.

Another advantage obtained by using composite powders results whenelements that are strong carbide formers are selected for vapor phasesintering with e.g. zinc. In the case of elements such as titanium,columbium, tantalum and others, some reaction between these elements andthe carbon/ graphite molds is to be expected. If such formation ofcarbides is determined to be detrimental to the articles formed byvapor-sintering, then suitably coated composite powders can be used toreplace them since the coating on the composite powders, even whenmodified with e.g. zinc, acts as a barrier layer between the corematerial and the carbon or graphite.

Nickel, cobalt and copper are the principal metal coatings presentlybeing made available on a variety of core materials. In addition tothose named above, other core materials being offered are: alumina,chromia, zirconia, silica, vanadia, titania, SiC, TiC, Cr3C2, TiB2, VBZ,CrB2, MoSi2, Si3N4 and TiH2.

It will be understood that the production of composite powders of thekinds described above forms no part of the present invention and thatthe foregoing discussion merely relates to the scope of the termcomposite powders as used in this specification.

Reverting now to the main heading above of Particular Alloys and MetalCompositions, it will be understood that when composite powders are usedin whole or in part as starting powders, the practice of the presentinvention can yield finished articles having a broad scope as far asultimate chemical composition is concerned. Thus the metal powders whichcan be used alone or with composite powders as starting powders includethe ductile heavy metals named above, columbium, tantalum, vanadium,manganese, titanium, zirconium, and alloys and/or mixtures in which oneof the foregoing metals forms the base; i.e. constitutes the larger orlargest amount by weight in the total. All such starting metal powdersare amenable to vapor-sintering, particularly with zinc.

It should now be clear that when practicing the vaporsintering processto produce particular finished compositions, the retorting may in somecases need to be carried out at superatmospheric pressure and in othersat subatmospheric pressure. Considering zinc as a typical vaporizablemetal, for alloy systems wherein the melting point of an element, alloyor component of a composite powder to be alloyed with zinc isconsiderably above l663 F., the use of pressure in excess of atmosphericcan be considered. By the use of a pressurized system, the vaporizationof zinc is impeded until the temperature of the system is sufficientlyhigh to insure the formation of alloy phases between zinc and theelement that will aid integration. When a component of the system has amelting point below a selected vapor-sintering temperature, and meltingis to be avoided, then the retort can be operated at an appropriatesubatmospheric pressure, whereby to secure an adequate vapor pressure ofzinc at a temperature low enough to avoid said melting. If melting of acomponent of a batch of starting material is not detrimental, then ofcourse the parameters of pressure, temperature and time can be adjustedas desired.

The following examples illustrate the principles of my invention andinclude the best modes presently known to me for practicing theinvention in accordance With those principles.

Example 1 Articles of jewelry are prepared from nickel powder (82% minus325 mesh) and zinc vapor, using starting proportions of 8 oz. nickelpowder to 2.5 oz. zinc powder mesh). The molds for the articles wereprepared in the way described above. in connection with FG. I, usingpatterns made by pouring molten polyethylene glycol and by using amolding mixture composed of 3 parts by weight of graphite powder (minus35 mesh), 1 part by weight of commercial furfuryl alcohol resin, and1.5-2%

by wt. (based on resin) of commercial acid converter. The converter (asolid powdery material) was mixed thoroughly with the graphite powder,after which the liquid resin was added and thoroughly mixed with the drypowders until a uniform plastic mass was secured. This plastic mass wasthen tamped around the patterns, levelled off and otherwise treated asdescribed above. The so-formed molds were fired at l200 F. forcarbonization and elimination of the patterns. The resulting molds arefilled with the nickel powder and stacked with baille plates, etc. inthe manner explained in connection with FIG. ll, the zinc powder beinghere introduced in the proportions noted above. The resulting assemblyis then enclosed in an expendable steel can and heated to l750 F. in thefurnace of FIG. III in a period of three hours of continuous temperaturerise, and cooled immediately while still in the furnace. The moldedmetal articles are recovered, wire-brushed, sanded lightly and polished.The polished surfaces are silver-like in appearance and the articles arewholly of merchantable quality. Chemical analyses taken at random ofentire cross sections of l/s" thick parts established the articles tohave a composition of 32-35% zinc, balance nickel.

Example 2 Example l is repeated except that patterns are made of acrylicplastic, minus 325 mesh copper powder is substituted for the nickelpowder, and the heating cycle is terminated when a temperature of l550F. is reached. The finished articles, when polished, exhibit the colorof yellow brass, and can be soft-soldered to brass wire, jewelryfindings of brass, steel wire, etc. Chemical analysis of an entire crosssection of an 1A" thick part shows the article to have a composition of33% zinc, balance copper.

Example 3 An ornamental nameplate measuring about 2 x 6 x 1/4" is madesubstantially in the manner of Example 1 except for the followingvariations:

(a) A nameplate pattern is used,

(b) In building up the mold assembly for retorting, the total zincpowder of Example 1 is divided into two equal portions, one portionbeing distributed between the lower baffle plate and the filled mold,while the other portion is distributed between the filled mold and theupper bale plate.

The finished nameplate, when analyzed chemically, is found to have thesame percentages of zinc and nickel as in Example l.

Example 4 A molded amalgam part is prepared in the manner described inExample 1 by using silver powder as the starting powder and by usingmercury as the vaporizable metal. In the vapor-sintering treatment, amaximum temperature of 1200" F. is employed, with a starting Weightratio of 3:1 between the silver powder and the mercury. The mercury issprinkled on the graphite powder in the same general way as the zincpowder is distributed in Example 1. The resulting molded article is asilvery white alloy containing 20-30% mercury.

Example 5 Plates adapted for use as anti-fouling ship-bottom attachments are made from copper powder as the starting powder and fromarsenic as the vaporizable metal. The process is carried out essentiallyin the manner described in Example 1, the weight ratio of arsenic tocopper being 1:4, and the maximum furnace temperature during thevapor-sintering treatment 'being l200 F.

If desired, similar plates which are laminated with copper can beprepared by inserting a flat sheet of copper, e.g. copper foil, on thebottom of each mold cavity before pouring in the copper powder. In suchcase, the vaporsintering treatment bonds the copper/ arsenic layer tothe 1 5 foil, and the said layer can be made very thin, eg. 0.050" orless.

Example 6 An abrasive block measuring about 1 x 3" x l/s is prepared bystarting with nickel-coated silicon carbide (composite) powder andvapor-sintering it with zinc vapor. A weight ratio of Zinc:compositepowder of 1:3 gives a finished block having a suitable wear rate whenused on soft metals such as electrical contact points.

For preparing tougher abrasive articles adapted for use on iron, stone,etc., the starting powders can consist of a mixture, e.g., iron powdermixed with the composite powder, in proportions commensurate with thewear rate desired under the intended conditions of use.

Example 7 Molded copper/zinc articles containing about zinc by weightare made in the manner described in Example 1 by starting withformulations of copper powder which have been carefully graded inrespect to particle size distribution so as to give optimum packing whenpoured in the mold cavity; the formulations are as follow:

For irregular particles: Percent -150 +200 mesh 20 400 +200 mesh 80 Forspherical particles: Percent -100 |150 mesh 60 -325 mesh 40 The finishedarticles are found to be well integrated, albeit still porous, by thesmall amount of zinc which permeates the starting powders during thevapor-sintering treatment.

Example 8 Jewelery items are prepared in the manner described in Example1 except that the starting nickel powder thereof is replaced in totowith an equal weight of nickel silver powder (i.e. a 65% Cu/l8% Ni/l7%Zn alloy).

Example 9 In this example, the principles of the present invention areapplied to prepare a vapor-sintered Ni/Zn coating on a sheet ofcolumbium, thereby to secure a coated sheet better able to withstand theintense heat of reentry experienced on, e.g., leading edges of airfoilson space craft. Reference should be made to FIG. V o-f the drawingswhere it is shown that the retort in this instance is a welded steelenvelope made from a pan 24a and a lid 24b. The envelope tightlyencloses an assembly consisting of (a) a base graphite plate 25 carryinga weighed layer 26 of 35 mesh zinc powder, (b) a porous graphite spacingplate 27, its upper surface having been given a wash coat of bentonite(to prevent formation of columbium carbide by contact between thegraphite and the columbium sheet), (c) the columbium sheet 28 which isto be coated, (d) a layer 29 of fine nickel powder (minus 200 mesh orfiner), and (e) a top poro-us graphite plate 30. Preferably such anassembly is made up by starting with an open drawn steel pan 24a of e.g.rectangular configuration so dimensioned as to be a close fit around theabove assembly. After the assembly has been stacked within the pan, theflat steel lid Zlib is put into place and welded at its edges to thepan. An evacuation port (not shown) can be included in either the pan orthe lid to permit withdrawal of residual air within the envelope. Suchwithdrawal is not usually necessary and when practiced no strictdegassing treatment is needed. After the internal pressure has beensuitably lowered, the evacuation port can be welded shut in theconventional manner.

For the intended purposes of the Ni/Zn coating, the starting ratio ofnickel to zinc can Vary on a weight basis from about 1:1 to about 1:20,the ratio being chosen in accordance with the expected serviceconditions to be encountered. It will be understood that whilezinc-coated columbium has been found to be more resistant than barecolumbium, the inclusion of high melting metal (here the nickel powder)in the coating serves to depress the vapor pressure of zinc and henceprolongs its service life, thereby giving the columbium protection for alonger period of time. Moreover, the nickel component in the presentcoating raises the melting point, all to the same end.

The enveloped assembly is heated, as Will be understood, to vaporize thezinc of layer 26, thereby to vaporsinter the nickel of layer 29 toitself and to the contacting surface of the columbium sheet 28. Themaximum temperatures to be attained in such heating' vary with the Ni/Znratios being used. For the ratio of 1:20, a temperature of about 1700 F.is used, while for the 1:1 ratio the maximum temperature is about l900F. A straight line relationship can be used for intermediate ratios.

It should be understood that other metals besides columbium can becoated with vapor-sintered Ni/Zn coatings for similar protectiveeffects, such as columbium-base alloys, vanadium-base alloys, and otherhigh-melting metals and alloys which form intermetallic compounds withZinc and/or nickel. The nickel in such coatings can be replaced in wholeor in part with cobalt.

It will be noted in FIG. V that the nickel layer 29 rests on the uppersurface of the columbium sheet 28. This is to insure firm contactbetween the two. If it is desired to coat both surfaces of the columbiumsheet in one operation, the needed contact between nickel powder and theunder surface can be secured by painting the under surface (or both, ifdesired) with a suspension of nickel powder in a nitrocellulose/ethylacetate binder solution, and drying the applied coatings before makingup the assembly in the enclosing envelope.

In final review of the invention, it should be noted that while l havedescribed the preparation of various articles by my vapor-sinteringprocess, various after-treatments can be applied to said articles, ifdesired. For example, an article molded by the process described inExample l can be re-run in a second vapor-sintering treatment with asmall added amount of, eg., zinc, thereby to increase the amount of zincat and close to the surface of the article. Such a treatment can becarried out so as to effectively seal the surface, thereby making theso-finished article amenable to electroplating. For a like purposearticles produced by the process of, e.g., Example l can be dipped intomolten zinc, whereby to seal the surface. Again, one can subject anarticle prepared by, e.g., Example 1 to localized heat sufficient tocause slight melting of the whole surface of the article or of onlydiscrete selected portions thereof. High frequency inductive heating,with or without selective cooling probes, can be used effectively forthis purpose. On jewelry items in particular, such melting at thesurface (in whole or in part) can be useful in modifying sharp edges andpoints of detailed design into softly rounded edges and points, therebyto secure a decorative effect which is difficult to secure from the molditself. To the same end, however, a somewhat similar effect can besecured in the vapor-sintering treatment (as opposed to a specialafter-treatment) by intentionally raising the temperature (after thenormal treatment would otherwise be terminated) so as to causesuperficial melting of the vapor-sintered article. However, thisrequires very close control of furnace temperature and is more easilycarried out accurately with a retort holding only a few molds than witha retort holding a tall stack of molds. It will be understood that thisfeature is mainly one of using a furnace which has been designedcarefully so as to give uniform heat throughout the entire zone in whichthe retort is located.

Still another after-treatment (or an in-retort treatment) givingpleasing effects for some purposes is that of subjecting theas-vapor-sintered article to an intentional thermal gradient sufficientto cause melting in a selected rcgion or portion, eg., at one end. lnthis way sharp molded features are retained at the unmelted end whileslumping, contraction and loss of detail is continuously graduated asthe melted end is approached.

Having now described my invention, what I claim is:

1. The process which comprises the steps of (I) heating an almostgas-tight retort confining under non-oxidizing conditions an assemblycomprising (a) a mass of loose uncompacted metal powder poured into andgiven a desired shape by the supporting surfaces of a top-lling cavitycarried in a porous mold member composed essentially of non-metallicrefractory material which is nonoxidizing to said metal powder, and (b)a predetermined mass of vaporizable metal disposed outside of saidcavity and out of physical contact with said loose metal powder in saidcavity, to an elevated temperature (1) generating non-oxidized metallicvapor of said vaporizable metal within said retort but (2) below themelting point of said metal powder, (II) continuing said heating undersaid non-oxidizing conditions until metal vapor has acted on said massof loose metal powder to vapor-sinter the latter into a unitary,self-sustaining metallic body having the shape imposed on said metalpowder by said cavity, and (Ill) cooling said assembly under conditionspreventing oxidation of said unitary metallic body, thereby to recoversaid body; said vaporizable metal being selected from the groupconsisting of magnesium, zinc, cadmium, arsenic, rubidium, cesium,potassium, sodium and mixtures thereof, and said loose metal powderbeing selected from the group consisting of iron, cobalt, nickel,manganese,

titanium, zirconium, columbium, vanadium, copper, silver, gold alloys inwhich one of the foregoing elements constitutes the base, compositepowders, and mixtures of the foregoing.

2. The process as claimed in claim 1 wherein the starting weight ratioof vaporizable metal to total metal powder in said loose mass is atleast 1:20.

3. The process as claimed in claim 2 wherein said porous mold member iscomposed essentially of carbon, wherein said retort has a small ventopening, and wherein said heating is accomplished by applying heatexternally of said retort.

4. The process as claimed in claim 3 wherein said loose metal powderconsists essentially of particles liner than about 325 mesh.

S. The process as claimed in claim 4 wherein said assembly occupiessubstantially all of the interior volume of said retort.

6. The process as claimed in claim 5 wherein said porous mold member isplatelike with substantially llat, parallel faces and is stacked in saidretort with other parts identified below and in the following sequencestarting from the bottom:

(a) a thin, substantially flat ferrous metal baflle plate carrying onits upper face a mixture consisting essentially of graphite powder andsaid predetermined supply of vaporizable metal in particulate form, and

(b) said porous mold member carrying loose metal powder in a cavitythereof,

and wherein said retort is made of thin ferrous metal and the ventopening is in the wall thereof which is above the stack of partsidentified above.

7. The process as claimed in claim 6 wherein each cavity in said moldmember has a vertical dimension of up to about one-fourth inch.

S. The process as claimed in claim 7 wherein the starting weight ratioof vaporizable metal to total metal powder in said loose mass is betweenabout 1:20 and 1:1, inclusive.

9. The process as claimed in claim 8 wherein said vaporizable metal iszinc.

10. The process as claimed in claim 9 wherein said loose metal powderconsists essentially o nickel.

11. The process as claimed in claim 9 wherein said loose metal powderconsists essentially of copper.

12. The process as claimed in claim 9 wherein said loose metal powderconsists essentially of a mixture of about equal weights of nickel andcopper powders.

13. The process as claimed in claim 9 wherein said loose metal powderconsists essentially of a mixture of about equal weights of nickel andiron powders.

References Cited UNITED STATES PATENTS 3,115,698 lf2/1963 Pierre 75- 201X 3,181,936 5/1965 Denny 75-223 3,214,270 10/1965 Valyi 75-201 3,313,6214/1967 Mott 75-201 X FGREIGN PATENTS 137,248 5/ 1950 Australia. 700,6077/1950 Great Britain.

22,103 10/1963 Japan. 146,046 1/ 1960 Russia.

OTHER REFERENCES Goetzel: Treatise on Powder Metallurgy, vol. I,Interscience Publishers, Inc., New Yorlr, 1949, pp. XXVI, 3.

CARL D. QUARFORTH, Primary Examiner.

BENIAMIN R. PADGETI", Examiner.

A. J. STEINER, Assiflant Examiner.

